1. Field of the Invention
The present invention relates to a method, system, and program for the remote copying of updates to primary and secondary storage locations subject to a copy relationship.
2. Description of the Related Art
Disaster recovery systems typically address two types of failures, a sudden catastrophic failure at a single point in time or data loss over a period of time. In the second type of gradual disaster, updates to volumes may be lost. To assist in recovery of data updates, a copy of data may be provided at a remote location. Such dual or shadow copies are typically made as the application system is writing new data to a primary storage device. Different copy technologies may be used for maintaining remote copies of data at a secondary site, such as International Business Machine Corporation's (“IBM”) Extended Remote Copy (XRC), Coupled XRC (CXRC), Global Copy, and Global Mirror Copy. These different copy technologies are described in the IBM publications “The IBM TotalStorage DS6000 Series: Copy Services in Open Environments”, IBM document no. SG24-6783-00 (September 2005) and “IBM TotalStorage Enterprise Storage Server: Implementing ESS Copy Services with IBM eServer zSeries”, IBM document no. SG24-5680-04 (July 2004).
In data mirroring systems, data is maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Primary and secondary storage controllers may be used to control access to the primary and secondary storage devices. In certain backup systems, a sysplex timer is used to provide a uniform time across systems so that updates written by different applications to different primary storage devices use consistent time-of-day (TOD) values as time stamps. Application systems time stamp data sets when writing such data sets to volumes in the primary storage. The integrity of data updates is related to ensuring that updates are done at the secondary volumes in the volume pair in the same order as they were done on the primary volumes. The time stamp provided by the application program determines the logical sequence of data updates.
In many application programs, such as database systems, certain writes cannot occur unless a previous write occurred; otherwise the data integrity would be jeopardized. Such a data write whose integrity is dependent on the occurrence of previous data writes is known as a dependent write. Volumes in the primary and secondary storages are consistent when all writes have been transferred in their logical order, i.e., all dependent writes transferred first before the writes dependent thereon. A consistency group has a consistency time for all data writes in a consistency group having a time stamp equal or earlier than the consistency time stamp. A consistency group is a collection of updates to the primary volumes such that dependent writes are secured in a consistent manner. The consistency time is the latest time to which the system guarantees that updates to the secondary volumes are consistent. Consistency groups maintain data consistency across volumes and storage devices. Thus, when data is recovered from the secondary volumes, the recovered data will be consistent.
Consistency groups are formed within a session. All volume pairs assigned to a session will have their updates maintained in the same consistency group. Thus, the sessions are used to determine the volumes that will be grouped together in a consistency group. Consistency groups are formed within a journal device or volume. From the journal, updates gathered to form a consistency group are applied to the secondary volume. If the system fails while updates from the journal are being applied to a secondary volume, during recovery operations, the updates that did not complete writing to the secondary volume can be recovered from the journal and applied to the secondary volume.
A customer may want updates to a primary storage that are mirrored at a secondary storage to also be copied to a further remote storage as part of a remote copy session between the primary storage and remote storage. If a switchover or swap occurs from the primary storage to the secondary storage, then a new remote copy session must be established to copy the updates now being received at the secondary storage system to the remote storage system. Further, in current systems, if the remote session between the failed primary storage system and the remote site was asynchronous, a full copy of all data at the secondary storage site is made to the remote site if as a result of the failure some data was left in the failing primary storage system cache.
For these reasons there is a need in the art for improved techniques for handling failures in a mirrored environment.